Delivery of liquid precursors to semiconductor processing reactors

ABSTRACT

This invention relates to methods and apparatus for delivering liquid precursors to semi-conductor processing apparatus. The liquid precursor delivery system is generally indicated at  10  and includes a source  11,  an inlet tube  12,  a two-way valve  13,  a pump assembly  14,  an outlet tube  15,  a shut-off valve  16  and a flash evaporator  17.  The pump assembly  14  is in the form of a syringe or variable volume pump and is controlled by a combination of a step motor  27  and a linear encoder  30.  The arrangement is such that unused liquid precursor can be returned to the source.

[0001] This invention relates to methods and apparatus for deliveringliquid precursors to semiconductor processing reactors.

[0002] There is a good discussion of various prior art proposals fordelivering such precursors in the preamble of U.S. Pat. No. 5,620,524and the reader is referred to this. In general a liquid precursor willbe a gas at the low pressures, which typically exist in the reactor(e.g. 1 torr), but at atmospheric pressure (15 torr) they will be intheir liquid state. This gives rise to practical delivery andmeasurement problems, which are exacerbated by the highly reactivenature of many of these precursors, which limit the choice ofengineering materials that may be used. Most liquid precursor deliverysystems, which are used in wafer processing, are of one of the followingthree types:

[0003] 1. There is vaporisation of the liquid and the use of a gas massflow controller.

[0004] 2. Use of a liquid mass flow controller.

[0005] 3. A carrier gas is passed through a liquid in a “bubbler” tovaporise some of the liquid in the carrier gas bubbles.

[0006] Whilst gas mass flow measuring instruments are widely used, theypresent problems when the liquid has to be vaporised for mass flowmeasurement. A large reservoir of vaporised liquid is required, whichincreases the residence time of the vaporised liquid at a raisedtemperature. For peroxide, which is a useful precursor, decompositionmay commence before it reaches the process chamber. Liquid mass flowcontrollers are capable of giving good flow control but are liable toinstabilities, believed to be caused by bubbles forming in the mass flowcontroller whilst in its idle state and the cold liquid cooling thecontroller can cause calibration drift. Bubblers require a carrier gasand the lower the vapour pressure, the less liquid that is taken up asvapour by the bubbles. Therefore, for low vapour pressure liquids, ahigh flow of carrier gas is required and that may be incompatible withthe process.

[0007] As alternatives to these approaches, an arrangement similar tothat shown in U.S. Pat. No. 5,620,524 has been tried comprising apositive displacement pump in combination with a narrow bore outletpipe, but as well as difficulties in selecting appropriate materials forsuch pumps and other parts, flow variations are experienced due tovariations in bore of the narrow bore pipe, necessary to buffer the lowpressure chamber from the pump. Such arrangements are susceptible tovariations because the flow restriction of the narrow bore pipedominates as the pump is essentially only a pressurisation device. Theresults of this method are referred to as “standard” in FIG. 3 below.

[0008] A simpler version of this system can be used if a pre-set orfixed flow is acceptable. The system then entirely depends on the actionof ambient pressure upon the liquid to replace the use of the pump andthe flow restricting tubing to control flow. This system makes use ofthe fact that clean rooms are pressure controlled environments atslightly above atmospheric pressure. The system is still howeverdependent on the internal bore diameter of the pipe being accurate andin suitably compatible materials this is not often the case. Specificsystem calibration can therefore become necessary and different pipelengths may need to be used depending on the actual bore diameter. Theresults of this method are referred to as “fixed flow” in FIG. 3 below.

[0009] U.S. Pat. No. 5,620,524 describes an alternative usingout-of-phase piston pumps, but it relies on the system knowing variouscharacteristics of the precursor and monitoring the temperature andpressure of the precursor in order to deliver a desired mass ofprecursor.

[0010] U.S. Pat. No. 5,098,741 describes the use of a positivedisplacement pump to feed a liquid precursor to a CVD chamber, but thesystem requires the use of a pressure metre controlling a variableorifice valve to overcome the problem of dissolved gases.

[0011] From one aspect the invention consists in apparatus fordelivering a liquid precursor to a semiconductor processing reactorcomprising a source of a liquid precursor, a volume calibrated positivedisplacement pump drawing liquid precursor from the source via an inputpath and for delivering, along an output path, (preferably without anypressure control and/or pulsing) a known volume of liquid precursor tothe reactor. Either just prior to the reactor or within the reactor theliquid precursor may be evaporated, for example by a flash evaporator.

[0012] The apparatus may further include a valve for connecting the pumpto the input path or the output path. The apparatus may further comprisecontrol means for controlling the pump and the valve whereby the pumpdraws liquid in, whilst the reactor is not processing with thisprecursor. Additionally or alternatively the apparatus may furtherinclude control means for controlling the pump and the valve whereby thepump returns any undelivered precursor to the source. This recucling,particularly when combined with the pump only being charged when needed,significantly reduces the presence of dissolved gases.

[0013] The pump may be in the form of a syringe pump.

[0014] The source may include another reservoir of liquid precursor andpreferably the source may include at least two bottles or reservoirs andmeans for automatically connecting one bottle or reservoir to the inletpath as the other becomes empty.

[0015] The apparatus preferably includes a linear encoder controlledstepper motor for driving the pump to deliver the known volume.

[0016] From another aspect the invention consists in a method ofdelivering a liquid precursor to a semiconductor processing reactorincluding:

[0017] 1. Delivering a liquid precursor, along an input path, into avolume calibrated positive displacement pump from a source of liquidprecursor;

[0018] 2. Delivering, along an output path, preferably without pressurecontrol and/or pulsing a known volume of liquid precursor to thereactor.

[0019] The liquid precursor may be evaporated either just before itreaches the reactor or within the reactor.

[0020] This may further include permanently connecting the pump to theinput path and the output path. Additionally or alternatively, aftereach volume delivery, the pump may return any remaining liquid to thesource, before drawing a fresh charge.

[0021] Although the invention has been defined above it is to beunderstood it includes any inventive combination of the features set outabove or in the following description.

[0022] The invention may be performed in various ways and specificembodiments will now be described, by way of example, with reference tothe accompanying drawing in which:

[0023]FIG. 1 is a schematic view of a liquid precursor delivery system;

[0024]FIG. 2 is a graph indicating flow rate from the system; and

[0025]FIG. 3 is a further graph indicating variations in depositionrates using a variety of liquid precursor delivery systems.

[0026] A liquid precursor delivery system is generally indicated at 10in FIG. 1. The system 10 includes a precursor source generally indicatedat 11, an inlet tube 12, a two-way valve 13, a pump assembly, generallyindicated at 14, an outlet tube 15, a shut off valve 16, and a flashevaporator 17.

[0027] The liquid precursor source 11 comprises a pair of bottles 18, 19contained within a temperature controlled chamber 20, level sensors,schematically indicated at 21 detect the level of liquid in therespective bottles 18, 19. The bottles 18, 19 are connected to a valve22 and thence to the inlet tube 12. The valve 22 is arranged to connectone or other of the bottles 18, 19 to the tube 12.

[0028] The pump assembly 14 is of the type generally known as a syringeor variable volume pump, such as, for example, made by Lee Products intheir LPV series. Such pumps have a syringe 24 comprising a tube 25 anda piston 26. The piston is linked to a stepper motor 27 and in generalmeans are provided for detecting the linear movement of the piston todetermine the volume of liquid expelled from the syringe.

[0029] In the illustrated arrangement the stepper motor 27 drives a leadscrew 28 which in turn linearly moves a carriage 29 that is connected tothe free end of the piston 26. The linear position of the carriage 29 isdetected by a linear encoder 30. As the position of the carriage 29 isrevealed very precisely by the linear encoder 30, the position of thepiston 26, and hence the volume of liquid displaced, can be very tightlycontrolled and without relying on the pump being operated over aprecisely set time period. Further the whole arrangement enables liquidto be expelled at high pressure.

[0030] The pump assembly 14 and valves 13, 16 and 22 are all under thecontrol of a control circuit 31, which is in turn responsive to thelinear encoder 30 and the level sensors 21, as well as other processinginformation from the associated reactor.

[0031] In use the stepper motor 27 initially draws the carriage 29 in adownward direction, which moves the piston in a withdrawing sense withinthe tube 25. With the valve 13 set to connect the pump 24 to the liquidsource 11, the liquid precursor can be drawn from one or other of thebottles 18, 19 depending on the position Of the valve 22. Once the pump24 is charged, the stepper motor 27 stops. The valve 13 is then switchedto connect the syringe pump 24 to the outlet tube 15 and at the rightmoment, in processing, the shut off valve 16 is opened and the steppermotor reverses the motion to drive the piston 26 back within the tube 25causing liquid precursor to be delivered into the flash evaporator 17.The volume of liquid expelled is directly related to the travel of thepiston 26 and hence of the carriage 29. This can be monitored by thecontrol circuit 31 via the linear encoder 30 and as soon as the desiredvolume is achieved, the stepper motor 27 is turned off. Thus it will beseen that very precise volumes of liquid precursor can be delivered athigh pressure into the evaporator without the problems which arise withother systems.

[0032] It is preferred that the syringe is filled with more than therequired volume to prevent any shortage in delivery. Conveniently, thenthe unexpelled liquid can be returned to the bottles 18, 19 via a valve13, tube 12 and valve 23 so that the liquid precursor can be kept in thebest conditions possible.

[0033] The provision of two bottles enables one bottle to be switchedover when it becomes empty, without any interruption in processing beingnecessary.

[0034] It is thus anticipated that wafers or batches of wafers can beintroduced into the reactor chamber, a precise volume of liquidprecursor delivered into the chamber via the flash evaporator 17 andthereafter the pump can be emptied and recharged ready for the nextwafer or batch.

[0035]FIG. 2 illustrates the linearity in flow rate that can be achievedwith the system of the invention and hence the accuracy of volumedelivery.

[0036]FIG. 3 illustrates the resultant benefits in terms ofrepeatability deposition rate. It will be seen that the syringe producesa very repeatable level of deposition using a hydrogen peroxide systemas compared with earlier delivery proposals.

[0037] The system has a number of advantages:

[0038] 1. It can be self primed at the beginning of a run and henceclear any lines of degraded hydrogen peroxide and bubbles from the lineand syringe.

[0039] 2. The syringe need only be filled immediately prior to theprocess avoiding degradation of the liquid precursor.

[0040] 3. The delivery rate can be fully programmable.

[0041] 4. The delivery rate can be fully monitored by the linearencoder.

[0042] 5. Any excess liquid precursor is returned to the source at theend of the process.

[0043] 6. Continuous running through the automatic bottle changeover canbe easily achieved. These benefits have been particularly noted withsuch precursors as hydrogen peroxide which do tend to degrade when notkept under suitable conditions. The delivery system also works very wellwith hydrogen peroxide being accurate to below 1 gram per minute. Flowrates can be easily changed without any significant loss of accuracysimply by changing the pump tubes and the matching pistons.

[0044] Experiments, which are reported, in our copending UK Patentapplication 0001179.1 entitled “Method and Apparatus for forming a filmon a substrate” using conventional systems and then a syringe pump todeliver cyclohexyldimethoxymethylsilane show that for at least someprecursors and processes acceptable results are only obtained when asyringe pump assembly as described is used. This disclosure isincorporated herein by reference.

1. Apparatus for delivering a liquid precursor to a semiconductorprocessing reactor including a source for the liquid precursor, a volumecalibrated positive displacement pump for drawing liquid precursor fromthe source via an input path and for delivering along an output path aknown volume of liquid precursor to the reactor.
 2. Apparatus as claimedin claim 1 further including a valve for connecting the pump to theinput path or the output path.
 3. Apparatus as claimed in claim 2further including control means for controlling the pump and valvewhereby the pump draws liquid in, whilst the reactor is not processingwith this precursor.
 4. Apparatus as claimed in claim 2 or claim 3further including control means for controlling the pump and the valvewhereby the pump returns some or all of the undelivered precursor to thesource.
 5. Apparatus as claimed in any one of the preceding claimswherein the pump is a syringe pump.
 6. Apparatus as claimed in any ofthe preceding claims wherein the source includes a bottle or reservoirof liquid precursor.
 7. Apparatus as claimed in claim 6 wherein thesource includes at least two bottles or reservoirs and means forautomatically connecting one bottle or reservoir to the inlet path asthe other becomes empty.
 8. A method of delivering a liquid precursor toa semiconductor processing reactor including: (i) Drawing a liquidprecursor along an input path, into a volume calibrated positivedisplacement pump from a source of liquid precursor; and (ii)Delivering, along an output path, a known volume of liquid precursor. 9.A method as claimed in claim 8 including alternately connecting the pumpto the input path and the output path.
 10. A method as claimed in claim8 or claim 9 wherein is after each volume delivery the pump returns someor all of the remaining liquid to the source, before drawing a freshcharge.